function in namespace std does not name a template type

[bubble-07]

If I try to compile the python bindings using the recommended instructions on a stock Ubuntu 18.04 install [after installing necessary dependencies], I get the error mentioned in the title. Luckily, there seems to be an easy fix -- throw in an #include<functional> to the top of vptree.hpp.

[anselanza]

This is a handy fix. I wish I knew why it works, though?

I presume it's because the functional standard library includes the function class? as per https://en.cppreference.com/w/cpp/header/functional